The term surface active agent is often abbreviated as “surfactant,” which literally means “active” at a surface. Surface activity refers to a surfactant’s ability to lower the free energy of surfaces and interfaces. To put it another way, they reduce the surface and interfacial stresses. But this is not a unique quality. When added to an aqueous solution, the majority of water-soluble organic compounds reduce surface tensions and interfacial tensions, but the impact is often far less than for surfactants.
The properties of surfactants and the interaction of surfactants with metal or metal oxide and the surrounding solution environment determine the level of adsorption. Understanding the behaviour of surfactants is critical to the optimal utilisation of surfactants in adsorption on a metal or metal oxide surface, aggregation and partitioning in the aqueous or organic phase, as well as the applications of surfactants in areas such as particle removal, pharmacy, and cosmetics.
Solid Surface Chemistry
Many surfactants can adsorb onto solids due to the electrostatic interactions between charged sites on the solid surface and the charged headgroups of the ionic surfactants. Most mineral surfaces in reservoirs can be assumed to be charged. There have been a variety of solids used in surfactant adsorption research. These solids have included “ideal” reservoir materials, such as alumina (Al2O3) and silica (SiO2), and “real” materials, such as kaolinite clays, river alluvium, and sandstones.
There are several crystalline phases of alumina arising from the different configurations possible for the aluminium and oxygen ions. The surface charge on alumina in contact with a surfactant solution arises indirectly from the crystal structure of the alumina. The most commonly used alumina in adsorption studies is alumina or corundum, which has a rhombohedral crystal structure comprising a hexagonal close-packed array of oxygen ions with aluminium ions on two-thirds of the octahedral sites. The other two forms of alumina are different phases, which have a cubic structure and a monoclinic structure.
Polymerizable Surfactants
Polymerisable surfactants are monomers that contain a chemical grouping, soluble in the continuous phase of the polymerization in their molecule. Obviously, to use in aqueous emulsion polymerization, the chemical group attached to the monomer has to be water-soluble. Similarly, for any non-aqueous system, the group has to provide stabilisation in whatever solvent is used as the polymerization medium.
The polymerizable surfactants are mostly used as co-monomers with other monomers in heterophase systems to provide stabilisation during and after polymerization. They can also be incorporated into functional groups for further reactions, such as crosslinking reactions or exchanges to other chemical groupings.
In free radical-initiated vinyl polymerizations, the mode and frequency of incorporation of the surface active monomer into the copolymer are very much dependent on the reactivities of the free radicals of the two monomers. At the same time, for effective and efficient stabilisation of the copolymer, the mode of placement of the stabilising moieties into the polymer chain is of utmost importance. Hence, the copolymerization reaction kinetics for their formation may not always provide the best possible structure for all monomer-polymerizable surfactant combinations.
